This invention relates to a selective charge removal system with which an image produced by a copier can be erased selectively by preliminarily removing charge from specified areas on its photosensitive body.
When the original document to be copied by a copier is too thick or somewhat smaller than a standard size and the copier is operated with its document cover in the open position, undesirable black marks may appear along the edges of the produced copy. On the other hand, some modern copiers are provided with capabilities for editing operations such as trimming, masking, centering and two-color copying by combining these modes of operations. To perform such operations, the user specifies an area on the original by means of numeric keys or a stylus pen and a light-emitting device provided to the copier selectively erases the electrostatic latent image formed on the photosensitive body. Such a light-emitting device typically comprises a large number of light-emitting diodes mutually screened apart by screening plates and disposed in a single column along a straight line perpendicular to the direction of transportation of copy sheets. The unsightly black marks along the edges and the like can thus be erased by such a device but a device with many light-emitting diodes arranged in a line is difficult to assemble and inconveniently costly. Moreover, the resolution of such a light-emitting device is determined by the pitch of the constituent diodes which is usually as big as about 2.5-2.7 mm and this means that charge cannot be removed very accurately.
In the case of a composite copying mode of operation in two colors by using the masking method, for example, the problem of resolution may arise as follows. With reference to FIG. 1 which shows the positional relationship between a specified area on an original document to be copied and a light-emitting device as well as the image which is copied, a rectangular area is specified by inputting the positions of its diagonal corner points such as P.sub.1 and P.sub.2 such that the areas inside and outside this rectangle are copied in different colors M and N, respectively. Along the boundary lines which are perpendicular to the direction of the light-emitting device (indicated by numeral 7), however, the two colors may overlap or there may appear an area with no color at all because of the inadequate resolution as shown by letter L.